Adjustment of the amplification of amplifiers



March 26, 1940. oux ET AL 2,195,152

ADJUSTMENT OF THE AMPLIFICATION 0F AMPLIFIERS Filed Aug. 4, 1937 P1- icm A 'b um/I41 mu 2504123 1 Mid/t Patented Ma'r. .26 1 940 ADJUSTMENT PATENT, OFFICE I ADH'LIFIERS Bernard .Rou x', Mario Sollima, and mm )1 a Gamzon, Paris, France Application August 4. 1937, Serial No. 1 57,390 I 1 c In France August 4 1%6 1 claim. (01. 179-471) The present invention concerns the adjustment of theamplification-of amplifiers of the type of those utilized for emission and reception in'wireless transmission, for the recording or reproduction of talking films and discs, and so on.

As a rule, ,this ad-justment'is performed by making use of a potentiometer which acts ina uniform manner upon all the frequencies that are amplified by the amplifier.

However, in some cases, it may be desired to separately adjust the amplification of the various of'tension being obtained by means of cursors or sliding contacts, the potentiometer in question being in series with a number of rheostats equal to the number of cursors "and the operation of every cursor being adapted to produce the displacement of the arm of the corresponding rheostat in such manner that the sum ofct he. values of the outputtensions of the various frequency bands, multiplied by respective coefficients, the whole being divided by the input tension of the system, remains constant. v

Other features of the present invention"wi.ll become app'arentjfrom the following description of an illustrative embodiment thereof. A preferred embodiment of the present inven tion will be hereinafter'descr'ibed, with reference to the accompanying drawing, only byway of example. In the drawing:

Fig. 1 is a diagram illustrating an arrangement according to the prior art.

Fig. z diagrammatically shows an of the present invention.

Fig. 3 is a detail view showing a specific arrangementaccording to the invention.

In o'rderto obtain separate amplification of In such an arrangement, the musical current.

oi a voltageequal to coming throughfi, passes embodiment through three separatepotentiometers P1, P2, P3.

and then through three amplifiers A1, A2, A3.

These amplifiers are provided with filters so that each of them permits the passage of only a bandof'the musical frequencies to be amplified:

to :wit,1 ow, medium, and high frequency bands.

The currents flowingout from these three amplifiers have, as their respective potentials VA, VM,

, VG, and they are mixed so as to have the "total potential V. i 3 It will be readily understood that, with this arrangement, the adjustment of P1 acts merely upon low notes, that of P2 upon medium notes and that P3 merely upon high notes.

In some applicationsthe three amplifierseach operates aseparate loud speaker. The mixing of the different bandsis then efiected in an accousticalmanner instead of being effected in an electric manner. [But the operation is analogous. For practical purposes, this arrangement has a very serious disadvantage:

The device above mentioned is chiefly'intended to ensure'the adjustment of the curve of response of the amplifier system curve, that is, to say the 1 adjustment of the-ratios of the three output 130- tentials VA, Viv and VG- Now, i'f the amplification of say A1 is reduced by acting upon P1, VA is reduced, which indeed gives the desired result. But, in addition, the total outpjut potential V isreduced.

The total amplification, which i'srepresented by the at F is reduced."

In other words, as soon as it is endeavoured to modify the curve of response, the total amplification is simultaneously modified.

' This defect is extremely disturbing, especially in applications in which the output level must be or film recording).

The present invention provides a multiple adjustment device which permits of varying at will the ratios of the potentials VA, VM, and VG with respect'to one another in such manner that the total: amplification 1 remains constant.

' kept 'within narrow limits (radio emission, disc In this case, the. expression constant total I amplification should. be clearly defined.

Whatever be the device that is employed, the total output potential V, for a given adjustment and a given input potential E, depends essentially upon the composition of the musical sound arriving through A.

Therefore, it will be endeavoured to obtain the following condition of amplification: i

ing a normal tune.

This condition will be more or less approximately obtained in the other'cases.

This condition may be expressed, as it will.

be hereinafter explained, by .a mathematical formula? Let E represent the electromotive force at the; input end. It will be assumed that the whole of plied with if the following relation is obtained,-

Whatever be the adjustment:

l, m and n being empirical coeflicients whichrepresent the relative valuesof the intensities of the a constant value three bands respectively, corresponding to low.

medium, and high notes, in the case of a normal orchestra, playing a normal tune.

The three coefiicients depend upon the limits chosen for the three bands of frequency. For practical purposes, it is generally. possible to sim-' plify by giving the sameimportance to low and high notes, so that the formula becomes:

(2) o+ gm+ VA But this is not absolutely necessary, and it will result from the following explanations that the invention also permits of complying to the general condition represented by equation (1 The invention comprises taking from a common potentiometer the potentials upon which VA, VM and VG depend, for a given value E (that is to say for instance the grid potentials of the amplifying tubes corresponding to each of the three frequency bands) by keeping stationary the point where the potential corresponding to VM is taken and by displacing on the potentiometer the cursors corresponding to both of the other po-' tentials. Each of these cursors is connected with a constant value v another cursor, the two last mentioned cursors tion as above set forth.

It will be assumed that it is desired to adjust the three potentials VG, MM, VA, applied to the grids of the three tubes G, M, A, from potential E supplied by the input tube L. i The problem to be solved consists in being able to adjust separately the ratios of the three potentials, VG, Vu

and VA, within the widest possible limits, while automatically complying with condition (2). I

For the sake of simplicity, it will be supposed that one of the three tensions, VM for instance, never needs to be quite. zero although .it can be extremely low, for instance, 20gb. below the two others. 1 i

Under these conditions, it suflices to have two cursor elements and P, to be able to adjust the a constant for the average of the sounds emitted for practical purposes by a normal orchestra playthree potentials. .;The input tube L feeds, current into a circuit including three resistances in series: two adjustable resistances or rheostats RA and Re, and a fixed resistance R, acting as potentiometer. The fixed resistance R is fitted with a stationary currentintake Q corresponding to the insertion of.

.a resistance I'M in the feed circuit of tube M,

and two adjustable intake cursors O and P corresponding to points the resistances of which are designated by variable quantities rA, re, feeding current to the respective tubes A and G. According tothe present invention, these two cursors are mechanically connected through suitable connecting means to the two cursors O and P' which adjust the useful valuesof resistances RA and Re. I

For instance, theseconnecting means will be such that cursors O and.O', on the one hand, and P and P, on the other hand, have the same displacement, whereas the three resistances RA, R6,; and R are arranged in such manner that the correspondingresistances r and RA onthe one hand, resistances rG and Be on the other hand vary according to a law such as is desired as a function of the displacements of the cursors. Let E be the difference of potential, in open circuit, in the secondary of the input transformer T; R0

the resistance of. the input circuit from the secondary, it will be assumed that the grids of the three tubes G, M and A are suitably polarized in such manner that theyfeedno current. Whatever be the value of, the current i, there are the following relations-zi As the value of rM is chosen as constant, it is clear that ratios depend respectively only upon the positions of cursors O and P. These cursors can therefore be graduated, it. so desired, in decibels of deadeningwith respect to VM. Point zero will be at Q and the values are positive on one side of Q and negative on the other side. 7 r i It will be noted that VG and VA can be made either separately or simultaneously equal to zero by placing the cursors at B. On the other hand VG and VA can be given separately'or simultaneously a maximum value by placing the cursors atP. i A

In this last'case, VM will not be zero, but it will be much smaller than VG and VA (for instance at most 20 decibels, that is to' say practically negligible) e The adjustment of one of the cursors does'not influence in any way the ratio of. the other potential and VM, so that the graduation in decibels keeps its value in all circumstances.

Consequently, the two operating members 0 the compensation I and P are sufficient for adjusting the relative value of the three potentials VG, VM and VA- which is an important simplification in many cases.

This arrangement further'permits of automatically complying with thecondition of compensation, which is: I

VG+mVM+VA mi+mm+mjm+mm+m E T E Furthermore, if it is desired to'give m any predetermined value, it isnecessary, according to the invention, to shunt potentiometer R-by means of a variable resistance S.

As a matter of fact, the total current is Now, it will be understood that, as a come -quence of the mechanical connection between cursors P and P, resistances re and Ravary simultaneously. It is therefore possible to arrange the windings in such manner that-these resistances are proportional to one another, whatever be the position of the cursors. Therefore, in this 7 case Re is equal to Kre, G being a constructional constant. I

In a similar manner RA may be considered as It will be seen that in order to have a constant value of a, independent of the adjustable value of re, and m, it suffices to comply with the following relation:

As all the terms of this equation are constant, 7

it is easyto choose S and R in such manner as to comply with this condition, so that a has a constant value. l I

It will be noted that, by modifying the value of S, the condition that the amplification re mains constant is maintained, but with a dif-* ferent value of m, that is to say with a variable importance of the medium frequency band. This permits of experimentally adjusting the value of.

m as well as possible in accordance with the limits chosen for the three bands of frequencies. For practical purposes, it is advantageous to arrange theoperating members in such manner that their position is easily visible (Fig. 3). 1 Arm A controls cursors P, P.

I Arm G controls cursors O, 0. These arms move in front of .dials :U, X, graduated in decibels.

When G, M and A are the three low, medium and high frequency bands of a triple amplifier, the position of the arms reproduces, at least approximately, the curve of transmission obtained for the adjustment that is considered, which greatly facilitates the operation of the appavratvls.

Although, in the preceding description, -we have referred merely to an amplifier relating to three bands of frequencies, which is the most frequent case for practical purposes, it should be well understood that the present invention is applicableto all cases, whatever may be the number .of bands of frequencies that are being considered.

Furthermore, we might provide one or several fixed intakes on the potentiometer for one or several' bands, or,-on the contrary, provide no fixed intake.

- Of course, .the number of rheostats in series with the potentiometer will be always equal to the I number of movable current intakes of the potentiometer, the cursors of these movable intake elements'driving through suitable transmission means the various cursors of; the rheostats respectively. I I I I While in a generalmanner, in the above description, we have disclosed what is now deemed to be practical and efiicient embodiments of the I invention, it should bewell understood that it is not intended to limit the scope of the" patent I thereto as there might be changes made in the arrangement, disposition, and form of the parts without departing from the principle of the present invention as comprehended within the scope of the appended claim.

What we claim is: i A system of the type described combination, three amplifiers, each corresponding to a band of sound frequencies, a source of electric current connected to supply electric current to said amplifiers, a potentiometer COIIHECt. ed between. said source and said amplifiers,

cursors on the potentiometer for connecting said potentiometer with said amplifiers, a plurality of rheostats, equal in number to the number oi. cursors electrically connected in series with the potentiometer, manipulating members for the rheostats, respectively, andmechanical means interconnecting said manipulating members with the respective cursors, the manipulation of each entrance potential of the whole, and Z, m, n

empirical coefficients representing the relative values'of the intensities of the three-said. sound frequency bands.

. BERNARD oux. MARIO SOLLIMA.

ROBERT GAMZON.

comprising, in 

